We have produced specific antibodies against an actin cross-linking protein isolated and purified from sea urchin egg actin gels. Indirect immunofluorescence staining localizes the cross-linking protein in the actin filament cores of sea urchin coelomocyte filopodia and in the filament cores of egg microvilli in fertilized eggs. This protein appears to be directly involved in the redistribution of actin filaments into actin bundles. We are currently studying the changes in the distribution of the cross-linking protein before and after fertilization using a radioimmunoassay. Further work on actin gelation in crude sea urchin egg extracts indicates that the calcium sensitivity of gelation is due to a block in actin filament assembly. Work is proposed to further characterize this calcium sensitive filament assembly and identify the control proteins involved.